Wind turbines (WTG) above 1 MW normally use individual pitch regulation to reduce load forces and to improve the cost effectiveness of the WTG. Asymmetrical loadings across the rotor are responsible for a significant contribution to fatigue loads and in principle it is possible to reduce these loads by controlling the pitch separately according to the conditions experienced by each blade. The pitch angle will normally have a position from approx 5 to 90 degrees. To decide whether the angle of attack has to be adjusted, some kind of load measurement system on the blades or on the shaft must be integrated in the WTG. Hereafter the controller can generate the estimated pitch demand signals for each pitch control system.
Different measurement sensors are used today. Strain gauges in the blades roots are used. Also Accelerometers in the blade tips are also well known. Optical fibers such as Smartfiber and Insensys can be mounted on or in the blades. These detectors are all rotating with the blades and measuring results have to be communicated from the rotating rotor towards a non rotating control system. Disadvantages for all these systems are the price, lifetime and reliability.
US20040151575A1 and US20040151577A1 concern both a method and apparatus for wind turbine rotor load control. Vertical and horizontal wind shears yaw misalignment and/or turbulence act together to produce asymmetric loading across a wind turbine rotor. The resultant load produces bending moments in the blades that are reacted through the hub and subsequently to the low-speed shaft. As a result, the main shaft and main shaft flange are displaced from their rest or non-aerodynamic load positions. The amount of shaft flange displacement is measured using one or more sensors. The output signals from the sensors are used to determine the magnitude and/or the orientation of the resultant rotor load. This information is used to effect the blade pitch change needed to reduce the load and thereby reduce fatigue and loading on various turbine components.
EP1243790A1 concerns a Wind generator, which has a rotor with at least one rotor blade for converting the wind flow energy into mechanical energy. An adjustment device adjusts the rotor blades individually, and a generator is coupled to the rotor to convert the mechanical energy into electrical energy. The instantaneous stress placed on sections of the wind generator installation is determined by a measurement system. A control system (8) determines the appropriate position of the rotor blade to provide this stress and adjusts it accordingly through the adjustment device.